This invention relates to a method for upgrading coal and, more particularly, a method for upgrading coal in-situ prior to mining.
Coal is graded by specific heat value, that is its energy output per unit weight. Excess moisture content substantially reduces the grade of the coal and lowers its market value accordingly. Further, the same excess moisture that lowers the grade of the coal also represents extra weight which increases the cost of transportation to the user. Thus, both the available sales price and the transportation cost provide incentive to reduce or eliminate excess moisture present in coal before it is mined.
Western subbituminous coal obtained from strip mining operations provides a great percentage of coal used in the United States. Here the seams of minable coal may be 50 to 100 feet thick, but the coal often has a high moisture content. In fact the coal seams are often within aquifers and, even after applying known draining techniques, a remaining moisture content of as much as 20-30%, and higher, is typical. Only about 1-3% of this moisture is surface moisture provided the coal is properly drained during mining and the rest remains as pore moisture, sometimes called inherent moisture, within the pores of the coal.
Past drying techniques have been based on processing the coal through fluid beds or other high temperature convection furnaces or conducting coal slurries through pressure vessels for combined temperature and pressure processing. However, the expense of such operations has limited their use. Further, such techniques are often only employed as preprocessing after the coal, together with its excess moisture, has already been transported to a site for use.